Cathode structure for vacuum tubes



W. HOFMANN Feb. 28, 1933.

CATHODE STRUCTURE FOR VACUUM TUBES Filed Aug. 1927 OXIDE COATINGInventor": Walter- Hofmann,

Hts Attorney Patented Feb. 28, 1933 UNITED STATES WALTER HOFMANN, FRUDOLSTADT IN THURINGIA, GERMANY, ASSIGNORTO GENERAL ELECTRIC COMPANY, ACQRPORATION NEW YORK:v I

GATHODE STRUCTURE FOR VACUUM TUBES I Application filed August 1, 1927,Serial No 209,858, and in Germany August 7, 1926.

The present invention relates to vacuum tubes and more particularly to acathode structure adapted to be employed in connection with X-rayapparatus.

When vacuum tubes and particularly tubes of the X-ray type are operatedat high voltages, electronic discharges sometimes occur from portions ofthe apparatus other than the cathode electrode, due to existing edgeswithin the tube or to surfaces having relatively small radii ofcurvature. Such discharges are harmful and are specially noticeable whenthe electrodes are spaced a short distance apart, such arrangement ofelec- 5 trodes being generally quite advantageous.

To overcome this difliculty, it has been heretofore proposed that withinthe area of the strong electric field, the portion of the cathodestructure, other than the part designed to emit electrons, be madeeverywhere slightly curved and that the surfaces of slight curvature besmoothened. 7

During the manufacture of vacuum tubes and especially high vacuum tubes,it is necessary during the pumping operation to bring all metal partswithin the tube to as high a temperature as possible. At such atemperature it is usually found that metal points and sharpened edgeswhich have been depressed in the polishing process become straightenedout and later in the operation of the vacuum tube send out electronicdischarges.

The capacity for emitting electrons varies with different materials andis inferior for all other materials than metal. In accordance with thepresent invention such parts of the cathode system in vacuum tubes whichare not intended to send out any electronic discharge are formed ofother materials than metal, or if formed of metal, refractory nonmetalcoatings are applied tothe metal. Coatings particularly well adapted foruse under such conditions are oxides, the structures of which aresubstantially constant at the high temperatures involved in theoperation of vacuum tubes and which do not vary under ion or electronbombardment. I have found that chromium oxide is particularly welladapted for this purpose.

The novel features which I believe to be characteristic of my inventionare set forth with particularity in the appended claims.

My invention itself, however, will best be understood from reference tothe following specification when considered in connection with theaccompanying drawing in which the single figure represents a sectionalview partly broken away of an X-ray tube embodying the features of myinvention.

Referring to thedrawing, I have indicated at 1, an evacuated receptaclehaving a reentrant stem 2 on which an elongated metallic cylindricalmember 3 comprising a portion of the cathode structure is mounted.Memher 8 at its outer end is slightly depressed,'as

indicated at 4, and within the depressed portion a cathode orelectron-emitting electrode 5 is mounted. The member 3 closely surroundsthe cathode element 5. A pair of leading-in wires 6 and 7 are connectedto the member 3 and electrode 5, respectively, so

that a potential may be supplied thereto in the normal operation of thetube. A cooperating anode or target 8 is mounted 0pposite the electrode5. To prevent electron Eerature conditions and electron andionbomardment and restricts the electron emission to the electrode 5alone.

What'I claim as new and desire to secure by Letters Patent of the UnitedStates, is

1. An electric discharge device comprising an evacuated receptacle whichterminates in a reentrant stem, a cathode and a cooperating .Such acoating re mains substantially constant under high temanode contained inthe receptacle, a support for the cathode comprising a metal tubesecured at one end to said stem and supporting the cathode atthe otherend, said tube being coated withrefractory oxide and having its edgesrounded whereby from the tube is precluded.

2. An electric discharge device comprising an evacuated receptacle whichterminates in electron emission a reentrant stem, a cathode and acooperating anode contained in the receptacle, a support for the cathodecomprising a. metal tube which fits over the reentrant stem and issecured thereto, 5 at the other end said cathode being mounted of thetube remote from the stem, said-tube being coated with chromium oxideand havlng its edges roundedwhereby electron emission from the tube isprecluded.

In witness whereof, I have hereto set my hand this 30 day of June, 1927.WALTER HOFMANN.

